Design for Small Cell Demodulation Reference Signal and Initial Synchronization

ABSTRACT

Described herein is a system with a first network element and a second network element. The first network element contains a processor configured to synchronize with the second network element; and maintain synchronization with the second network element. The first network element is a small cell eNB and the second network element is one of the following: a macro cell enhanced node-B (eNB); or a small cell eNB.

FIELD OF THE DISCLOSURE

The present disclosure relates to small cell demodulation referencesignal (DMRS) and initial synchronization.

BACKGROUND

As used herein, the term “user equipment” (alternatively “UE”) might insome cases refer to mobile devices such as mobile telephones, personaldigital assistants, handheld or laptop computers, and similar devicesthat have telecommunications capabilities. Such a UE might include adevice and its associated removable memory module, such as but notlimited to a Universal Integrated Circuit Card (UICC) that includes aSubscriber Identity Module (SIM) application, a Universal SubscriberIdentity Module (USIM) application, or a Removable User Identity Module(R-UIM) application. Alternatively, such a UE might include the deviceitself without such a module. In other cases, the term “UE” might referto devices that have similar capabilities but that are nottransportable, such as desktop computers, set-top boxes, or networkappliances. The term “UE” can also refer to any component that canterminate a communication session for a user. Also, the terms “userequipment,” “UE,” “user agent,” “UA,” “user device,” and “mobile device”might be used synonymously herein.

As telecommunications technology has evolved, more advanced networkaccess equipment has been introduced that can provide services that werenot possible previously. This network access equipment might includesystems and devices that are improvements of the equivalent equipment ina traditional wireless telecommunications system. Such advanced or nextgeneration equipment may be included in evolving wireless communicationsstandards, such as Long-Term Evolution (LTE) and LTE-Advanced (LTE-A).For example, an LTE or LTE-A system might be an Evolved UniversalTerrestrial Radio Access Network (E-UTRAN) and include an E-UTRAN node B(or eNB), a wireless access point, or a similar component rather than atraditional base station. As used herein, the term “access node” refersto any component of the wireless network, such as a traditional basestation, a wireless access point, or an LTE or LTE-A node B or eNB, thatcreates a geographical area of reception and transmission coverageallowing a UE or a relay node to access other components in atelecommunications system. In this document, the terms “access node” and“network element” may be used interchangeably, but it is understood thatan access node may comprise a plurality of hardware and software.

The geographical area of reception and transmission coverage provided byan access node may be referred to herein as a cell. Some cells may havesignificantly larger coverage area than others and may be referred toherein as macro-cells. Some cells may have significantly smallercoverage area than the macro-cells and may be referred to herein assmall cells. Small cells may also include micro-cells, pico-cells, andfemto-cells. In some embodiments, small cells may operate within thearea covered by a macro-cell.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a diagram of a portion of a wireless network.

FIG. 2 is diagram of a physical resource block (PRB) pair containing atransmission mode (TM) 9 demodulation reference signal (DMRS) pattern.

FIG. 3 is a diagram of a PRB pair containing DMRS pattern Alt 1.

FIG. 4 is a diagram of a PRB pair containing DMRS pattern Alt 2.

FIG. 5 is a diagram of a PRB pair containing DMRS pattern Alt 3.

FIG. 6 is a diagram of a PRB pair containing DMRS pattern Alt 4.

FIG. 7 is a diagram of a PRB pair containing DMRS pattern Alt 5-1.

FIG. 8 is a diagram of a PRB pair containing DMRS pattern Alt 5-2.

FIG. 9 is a diagram of a PRB pair containing DMRS pattern Alt 5-3.

FIG. 10 is a diagram of a PRB pair containing DMRS pattern Alt 6.

FIG. 11 is a diagram of a PRB pair containing DMRS pattern Alt 7.

FIG. 12 is a diagram of a PRB pair containing DMRS pattern Alt 8.

FIG. 13 is a diagram of a PRB pair containing DMRS pattern Alt 9.

FIG. 14 is a diagram of a PRB pair containing DMRS pattern Alt 10.

FIG. 15 is a diagram of a PRB pair containing DMRS pattern Alt 11.

FIG. 16 is a diagram of a PRB pair used by a first small cell in acluster.

FIG. 17 is a diagram of a PRB pair used by a second small cell in acluster.

FIG. 18 is a diagram of a PRB pair used by a third small cell in acluster.

FIG. 19 is a diagram of PRB bundling using three PRB pairs.

FIG. 20 is a diagram of PRB bundling using two PRB pairs.

FIG. 21 is a diagram of DMRS pattern association with PRB bundling.

FIG. 22 is a diagram of a macro-cell cell-specific reference signals(CRS) pattern.

FIG. 23 is a diagram of a small cell CRS pattern.

FIG. 24 is a diagram of a simplified network element.

FIG. 25 is a diagram of a user equipment.

FIG. 26 is a diagram of a processing component.

DETAILED DESCRIPTION

It should be understood at the outset that Although illustrativeimplementations of one or more embodiments of the present disclosure areprovided below, the disclosed systems and/or methods may be implementedusing any number of techniques, whether currently known or in existence.The disclosure should in no way be limited to the illustrativeimplementations, drawings, and techniques illustrated below, includingthe exemplary designs and implementations illustrated and describedherein, but may be modified within the scope of the appended claimsalong with their full scope of equivalents. Embodiments are describedherein in the context of an LTE wireless network or system, but can beadapted for other wireless networks or systems.

A small cell may exist within the area of coverage provided by amacro-cell. The small cell may transmit to user equipment at a differentfrequency than the macro-cell. Because small cells typically have lowtransmit power and thus small coverage area, and serve user equipmentwith low mobility, the radio channels change slowly in both time andfrequency. Therefore, less overhead may be required for transmittingdemodulation reference signal (DMRS) which are the reference signals(RS) for data demodulation. DMRS density may be reduced in the frequencyor time domain, or both.

In cases where the small cell transmits at the same frequency as themacro-cell, it is important to ensure synchronization between the smallcell and the macro-cell in order to enable coordinated multipointtransmission (COMP) and inter-cell interference coordination (ICIC). Thesmall cell may be configured to listen to a sync channel from amacro-cell or another small cell. The small cell may also configurespecial sub-frames in the downlink channel.

FIG. 1 is a diagram of a portion of a wireless network 100. The portionof the wireless network 100 contains a macro-cell 110 and a plurality ofsmall cells 120. While only one macro-cell 110 is depicted the wirelessnetwork may comprise many macro-cells 110 and many small cells 120. Thesmall cells 120 may exist within the coverage area of the macro-cell110, or the small cells 120 may be deployed outside of the macro-cell110 coverage area. The small cells 120 may be deployed in both indoorand outdoor scenarios as hot spots or to fill coverage holes of themacro-cell 110. Some of the small cells 120 may overlap in coverageareas. Each small cell 120 and macro-cell 110 may support one or moreuser equipment (not pictured). Each small cell 120 and macro-cell 110may be in wireless communication with the user equipment in theirrespective coverage area. The small cells 120 may be deployed tofacilitate offloading of UE traffic from the macro-cell 110. The smallcells 120 may also allow for increased data throughput to UEs, and anincreased per area throughput.

In some embodiments, the small cells 120 may operate at a differentfrequency than the macro-cell 110. For example, the small cells 120 mayoperate at the 3.5 GHz frequency band, while the macro-cell may operateat the 700 MHz frequency band. User Equipment in this embodiment may beconfigured to communicate using both 700 MHz frequency band and the 3.5GHz frequency band. The UEs may be served by both the small cells 120and the macro-cell 110. In some embodiments, the UEs may receive controlplane information from the macro-cell 110 and user data planecommunications from the small cells 120, or vice-versa.

In some embodiments, the macro-cell 110 and small cells 120 maycommunicate with supported UEs using the same frequency band. This maybe known as co-channel communication. Co-channel communication may beuseful when an operator does not have the additional spectrum availableto support the small cells on a different frequency band as that for themacro-cell.

In some embodiments, UEs with low mobility may be serviced by smallcells, while UEs with a high mobility may be serviced by macro-cells. Incertain embodiments where the small cell is servicing lower mobilityUEs, the root mean square (RMS) channel delay spread and Doppler spreadmay both be relatively smaller than UEs serviced in a macro-cell withhigh mobility, this may result in a relatively flat channel in thefrequency domain and less fluctuated channel along the time domain. Witha relatively flat channel in the frequency domain and less fluctuatedchannel along the time domain, it may be possible to reduce the densityof reference signals (i.e. DMRS) transmitted by the small cells withoutcausing a significant impact on communication performance of the smallcell.

In certain embodiments, DMRS may be configured differently for a smallcell in comparison with a macro-cell. For example, DMRS overhead may bereduced by reducing DMRS density along either the time or frequencydomain, or both. As another example, orthogonal DMRS assignments may beused for small cells deployed in a cluster. As used herein, orthogonalDMRS may include DMRS that are orthogonal in at least one of thefrequency domain, the time domain, the spatial domain, or the codedomain. Orthogonal DMRS assignments may require coordination betweensmall cells and may result in reduced interference as well as provideconvenience for interference estimation and cancellation for the UEswith an advanced receiver. The DMRS configuration may be signaledexplicitly to the UE through dynamic signaling or semi-static signalingor determined implicitly. The eNB may adaptively change the DMRSconfiguration and configure different DMRS patterns for different UEsserviced by the eNB. In some embodiments, physical resource block (PRB)bundling may be used by the small cell. With PRB bundling, differentDMRS patterns may be spread across several PRB pairs. As anotherexample, overlapping small cells may use shifted or complemented DMRSpatterns to reduce interference between the small cells.

In certain embodiments, the frequency band used by the small cell may bethe same as the frequency band used by the macro-cell, in this case, itmay be necessary to synchronize the small cell with the macro-cell. Thesmall cell may listen to a sync-channel from the macro-cell, or anothersmall cell using the same frequency. The small cell may configure aspecial sub-frame, for example a multimedia broadcast multicast singlefrequency network (MBSFN) sub-frame or almost blank sub-frame (ABS), andmonitor the CRS transmitted in the macro-cell or another small cellwhich it is synchronized with.

In 3GPP Rel-10, UE-specific DMRS was introduced for physical downlinkshared channel (PDSCH) demodulation in transmission mode (TM) 9. 8 DMRSports are defined for TM 9 where a DMRS port may be an antenna port thatcarries a DMRS. The DMRS for DMRS ports {7, 8, 11, 13} are code divisionmultiplexed on a set of time frequency resources in a PRB, while DMRSfor DMRS ports {9, 10, 12, 14} are code division multiplexed on adifferent set of time frequency resources in the same PRB. The lengthfour orthogonal covering code (OCC), as defined in Table 1 below, isused on four resource elements (REs) along time domain of the PRB pairto multiplex four DMRS for DMRS ports ({7, 8, 11, 13}, or {9, 10, 12,14}). Each OCC code (i.e. [ w _(p)(0) w _(p)(1) w _(p)(2) w _(p)(3)])may spread on four resource elements in the PRB where index {0, 1, 2, 3}indicates the REs that it is going to spread onto. The OCC codes forDMRS at DMRS ports {7, 8, 11, 13} may be spread on one set of four REs,while OCC codes for DMRS at DMRS ports {9, 10, 12, 14} may be spread onanother set of four REs.

TABLE 1 OCC code used for multiplexing DMRS for DMRS ports Antennaport_(p) [ w _(p)(0) w _(p)(1) w _(p)(2) w _(p)(3)] 7 [+1 +1 +1 +1] 8[+1 −1 +1 −1] 9 [+1 +1 +1 +1] 10 [+1 −1 +1 −1] 11 [+1 +1 −1 −1] 12 [−1−1 +1 +1] 13 [+1 −1 −1 +1] 14 [−1 +1 +1 −1]

As used herein. a PRB pair may comprise one-hundred-sixty-eight resourceelements (REs). The PRB pair may comprise two PRBs, each PRB comprisingeighty-four REs. A RE comprises one Orthogonal Frequency DivisionMultiplexing (OFDM) symbol in time and one subcarrier in frequency. Asubframe may comprise two slots with seven OFDM symbols in each slot. APRB may comprise REs over one slot in time and 12 subcarriers infrequency. The terms “PRB pair” and “subframe” may be usedinterchangeably throughout.

FIG. 2 is a diagram of a PRB pair containing a TM 9 DMRS pattern 200.The x-axis may represent the time domain with each column representing aOFDM symbol. The y-axis may represent the frequency domain, with eachrow representing a sub-carrier. In TM9, each DMRS port set, DMRS ports{7, 8, 11, 13} 210 or DMRS ports {9, 10, 12, 14} 215, may require twelveREs 205 in each PRB pair 200 to transmit DMRS signals. Thus, in TM9,each PRB pair 200 may use twenty-four REs 205 for DMRS transmission.Typically, in TM9, a single DMRS port may use 12 REs for each DMRSsymbol, and up to 4 DMRS ports may be multiplexed across the 12 REs usedfor DMRS transmission. In addition, CRS port {0} 220 and CRS port {1}225, as well as CSI-RS ports {15-22} 230 are also transmitted in the TM9 PRB pair 200.

FIG. 3 is a diagram of a PRB pair containing DMRS pattern Alt 1 300. Toreduce the DMRS overhead, the DMRS of TM 9 DMRS pattern 200 transmittedin the second PRB of the PRB pair may be eliminated. In thisAlternative, only the first PRB of the DMRS pattern Alt 1 carries REs205 for DMRS transmission. DMRS pattern Alt 1 may keep the same DMRSdensity along the frequency domain as defined in TM9, but with reducedDMRS density in time domain. DMRS pattern Alt 1 may support four DMRSports, i.e. DMRS ports {7, 8} 305 and DMRS ports {9, 10} 310, whichmeans four layer MIMO transmission may be supported.

FIG. 4 is a diagram of a PRB pair containing DMRS pattern Alt 2 400. TheREs 205 that carry DMRS may be split between the first PRB and thesecond PRB. DMRS pattern Alt 2 may use two pairs of REs 205 in the firstPRB, one on each side of the PRB boundary and the middle pair of REs 205in the second PRB to carry DMRS signals. DMRS pattern Alt 2 may maintaingood balance on DMRS transmission along both time and frequency domains.DMRS pattern Alt 2, may support four DMRS ports, i.e. DMRS ports {7, 8}305 and DMRS ports {9, 10} 310, which means four layer MIMO transmissionmay be supported using DMRS pattern Alt 2.

FIG. 5 is a diagram of a PRB pair containing DMRS pattern Alt 3 500.DMRS pattern Alt 3 may use the middle pair of REs 205 in the first PRBand two pairs of REs 205, one on each side of the PRB boundary in thesecond PRB to carry DMRS signals. DMRS pattern Alt 3 may maintain goodbalance on DMRS transmission along both time and frequency domains. DMRSpattern Alt 3, may support four DMRS ports, i.e. DMRS ports {7, 8} 305and DMRS ports {9, 10} 310, which means four layer MIMO transmission maybe supported using DMRS pattern Alt 3.

FIG. 6 is a diagram of a PRB pair containing DMRS pattern Alt 4 600.DMRS pattern Alt 4 may reduce the number of REs used for DMRS along thefrequency domain, but maintain the number of REs used for DMRS along thetime domain. Thus, the pairs of REs that carry DMRS along frequencydirection are reduced from 3 to 2. DMRS pattern Alt 4, may support eightDMRS ports, i.e. DMRS ports {7, 8, 11, 13} 210 and DMRS ports {9, 10,12, 14} 215, which means eight layer MIMO transmission may be supportedusing DMRS pattern Alt 4, using sixteen REs 205.

FIG. 7 is a diagram of a PRB pair containing DMRS pattern Alt 5-1 700.FIG. 8 is a diagram of a PRB pair containing DMRS pattern Alt 5-2 800.FIG. 9 is a diagram of a PRB pair containing DMRS pattern Alt 5-3 900.Each of DMRS pattern Alt 5-1, DMRS pattern Alt 5-2, and DMRS pattern Alt5-3 may use one pair of REs along the frequency domain to carry DMRSsignals. DMRS pattern Alt 5-1, DMRS pattern Alt 5-2, and DMRS patternAlt 5-3 may each support eight DMRS ports, i.e. DMRS ports {7, 8, 11,13} 210 and DMRS ports {9, 10, 12, 14} 215, which means eight layer MIMOtransmission may be supported using DMRS pattern Alt 5-1, DMRS patternAlt 5-2, and 5-3, each using eight REs 205.

FIG. 10 is a diagram of a PRB pair containing DMRS pattern Alt 6 1000.FIG. 11 is a diagram of a PRB pair containing DMRS pattern Alt 7 1100.FIG. 12 is a diagram of a PRB pair containing DMRS pattern Alt 8 1200.FIG. 13 is a diagram of a PRB pair containing DMRS pattern Alt 9 1300.Each of DMRS pattern Alt 6, DMRS pattern Alt 7, DMRS pattern Alt 8, andDMRS pattern Alt 9 may support four DMRS ports, i.e. DMRS ports {7, 8}305 and DMRS ports {9, 10} 310, which means four layer MIMO transmissionmay be supported using DMRS pattern Alt 6, DMRS pattern Alt 7, DMRSpattern Alt 8, and DMRS pattern Alt 9, each using eight REs 205.

Table 2 below summarizes the DMRS overhead in the PRB pair arrangementsdescribed above. DMRS overhead reduction may reduce the DMRS overheadfrom 14% in TM 9 to between 4.76% and 9.5% using the embodimentsdescribed above.

TABLE 2 DMRS overhead comparison TM 9 Alt 1 Alt 2 Alt 3 Alt 4 Alt 5-9 %DMRS 14% 7% 7% 7% 9.5% 4.76% per PRB pair

As described above, in Rel-10 TM9, length four OCC is applied along thetime domain to multiplex four DMRS ports transmitted in each PRB pair.Thus, four REs are required in the same subcarrier to support four DMRSports. Some of the embodiments described above only have 1 pair of REsalong the time domain and thus only support 2 DMRS ports per subcarrier.As such, some of the proposed DMRS patterns may support up to 4 DMRSports, for example, DMRS pattern Alt 1, DMRS pattern Alt 2, and DMRSpattern Alt 3, while others may support up to 8 DMRS ports, for example,DMRS pattern Alt 4, DMRS pattern Alt 5-1, DMRS pattern Alt 5-2, and DMRSpattern Alt 5-3.

FIG. 14 is a diagram of a PRB pair containing DMRS pattern Alt 10 1400.If the OCC is applied differently than it is applied in Rel-10 TM9 (i.e.only along the time domain), different DMRS patterns may be generated.DMRS ports {7, 8, 11, 13} 210 may use OCC code along both time andfrequency domain, similarly DMRS ports {9, 10, 12, 14} 215 may use OCCcode along both time and frequency domain. REs labelled with a 0 maycontain the OCC code with index 0, w _(p)(0), for the DMRS of each DMRSport as defined in Table 1. REs labelled with a 1 may contain the OCCcode with index 1, w _(p)(1), for the DMRS of each DMRS port as definedin Table 1. REs labelled with a 2 may contain the OCC code with index 2,w _(p)(2), for the DMRS of each DMRS port as defined in Table 1. REslabelled with a 3 may contain the OCC code with index 3, w _(p)(3), forthe DMRS of each DMRS port as defined in Table 1. As the channel doesnot change much in both time and frequency domain within such a smallsquare, orthogonality among different DMRS ports transmitted on the samesets of REs may be maintained.

FIG. 15 is a diagram of a PRB pair containing DMRS pattern Alt 11 1500.DMRS ports {7, 8, 11, 13} 210 may be transmitted in two sets of REs,with each set confined within a small square, in separate PRBs. LikewiseDMRS ports {9, 10, 12, 14} 215 may be transmitted in two sets of REs,with each set confined within a small square, in separate PRBs. REslabelled with a 0 may contain the OCC code with index 0 for the DMRS ofa DMRS port as defined in Table 1. REs labelled with a 1 may contain theOCC code with index 1 for the DMRS of a DMRS port as defined in Table 1.REs labelled with a 2 may contain the OCC code with index 2 for the DMRSof a DMRS port as defined in Table 1. REs labelled with a 3 may containthe OCC code with index 3 of a DMRS port as defined in Table 1. DMRSpattern Alt 11 occupies the same REs and has the same overhead as DMRSpattern Alt 4. However, PRB pair with DMRS pattern Alt 11 may have amore balanced DMRS density along both time and frequency domains and maybe less impacted by channel variations along both time and frequency,and therefore, may lead to better channel estimation performance.

The DMRS patterns described herein may use a subset of REs that are usedby the DMRS pattern typically used by TM9 for DMRS transmission. As usedherein, subset may mean fewer REs are used to transmit DMRS. Subset mayalso include shifting the location of the DMRS REs in a PRB pair alongeither time or frequency, or in some cases both time and frequency.Subset may also include stretching or rotating a TM9 DMRS pattern in aPRB pair along either time or frequency, or in some cases both time andfrequency. As used herein, subset may include DMRS patterns whose DMRSoverhead is less than a typical TM9 DMRS pattern DMRS overhead.

In some embodiments, small cells may be deployed in a cluster eachcovering a small area and supporting relatively few users. Small celleNB may be more cost-effective with reduced functionalities and hardwarethan a macro-cell eNB. One way to save costs may be to support fewertransmit antennas, for example, support 2 or 4 transmit antennas insteadof 8. Small cell deployment may require less planning efforts thanmacro-cell deployments (i.e. no towers etc.). Small cell clustering maylead to large overlaps in coverage and strong inter-cell interferenceamong small cells in a cluster. In some embodiments, the 8 DMRS portsmay be allocated to different small cells in a cluster in order toreduce interference.

FIG. 16 is a diagram of a PRB pair 1600 used by a first small cell in acluster. FIG. 17 is a diagram of a PRB pair 1700 used by a second smallcell in a cluster. FIG. 18 is a diagram of a PRB pair 1800 used by athird small cell in a cluster. DMRS ports {7, 8} 305 may be allocated tothe first small cell, DMRS ports {9, 10} 310 may be allocated to thesecond small cell, and DMRS ports {11, 12} 1805 may be allocated to thethird small cell. This type of allocation causes the DMRS ports in eachcell to be orthogonal to each other, and thus may reduce the inter-cellinterference on DMRS. Such orthogonal DMRS port assignment among smallcells may also facilitate inter-cell interference estimation andcancellation.

Transmitting several consecutive PRB pairs with the same precoding tothe same UE may be referred to herein as PRB bundling. PRB bundling maybe introduced to improve channel estimation performance. PRB bundlingmay include the eNB using the same precoding vector across the PRB pairsthat are bundled together. This may allow the UE to perform channelinterpolation over DMRS across bundled PRBs pairs. If such function iscombined with the DMRS design described herein, it may allow joint DMRSdesign to maintain effective channel estimation performance with reducedDMRS overhead.

As discussed above, for small cells the RMS delay spread may be small,which leads to a relatively flat channel along frequency domain.Therefore, coarse feedback granularity and resource assignmentgranularity would be suitable for use in small cells. Coarse granularityas used herein means that a larger sub-band size may be used for bothchannel feedback in the uplink and resource assignment in the downlink.For small cells, normally fewer UEs would be supported than macro-cells,but each UE will typically transmit and/or receive a larger payload.Increasing sub-band size may enable channel estimation to be conductedacross multiple PRB pairs jointly. For example, if three consecutive PRBpairs are assigned to a UE, channel interpolation may be used onreference signaling across the consecutively assigned PRB pairs toimprove the performance of channel estimation. Thus, the design of DMRSfor small cell may be selected based on the use of PRB bundling and mayresult in further DMRS overhead reduction.

In an embodiment, PRB bundling may be used, and three consecutive PRBpairs may be assigned to a UE. FIG. 19 is a diagram of PRB bundlingusing three PRB pairs 1900. As shown in FIG. 19, the first consecutivePRB pair may be configured using DMRS pattern Alt 5-1, the secondconsecutive PRB pair may be configured using DMRS pattern Alt 5-2, andthe third consecutive PRB pair may be configured using DMRS pattern Alt5-3. The channel interpolation and extrapolation may then be used overDMRS in each of the three PRB pairs to get the channel estimationperformance across all three PRB pairs. Further, the location of theDMRS ports changes along the frequency domain from one PRB pair to thenext, thus compensating for any interference resulting along thefrequency domain.

In another embodiment, PRB bundling may be used, and two consecutive PRBpairs may be assigned to a UE. FIG. 20 is a diagram of PRB bundlingusing two PRB pairs 2000. As shown in FIG. 20, DMRS pattern Alt 2 andDMRS pattern Alt 3 may be assigned to consecutive PRB pairs scheduled tothe same UE. This may result in balanced DMRS distribution across thePRB pairs and thus improve the overall channel estimation performancerelative to the case when a single DMRS pattern (i.e. only DMRS patternAlt 2) is repeated across PRB pairs.

An eNB may bundle PRBs based on their index in system bandwidth, wherethe bundling size may be based on system bandwidth. A similar mechanismmay be used in PRB bundling for a small cell and its association withDMRS patterns. For example, the PRB pairs in the system bandwidth may beindexed from lowest frequency to highest frequency over the whole systembandwidth. Within the bundled PRB, the same precoding shall be used byeNB. DMRS design as described herein may be incorporated by a one-to-onemapping relation between one DMRS pattern and one PRB pair. As anexample, DMRS pattern Alt 5-1 may be associated with a first PRB paircorresponding to the lowest frequency, DMRS pattern Alt 5-2 may beassociated with a second PRB pair, and DMRS pattern Alt 5-3 may beassociated with a third PRB pair, this association format may berepeated for the remaining PRB pairs from lowest frequency to highestfrequency. With such one-to-one mapping between DMRS pattern and PRBpair for a new UE (e.g. Rel-12 UE and beyond) assigned with a number ofconsecutive PRB pairs by an eNB, the UE may automatically determine theDMRS patterns for each PRB pairs it is assigned based on the PRB pairindex. For example, the UE may determine that DMRS pattern Alt 5-2, Alt5-1 and Alt 5-3, may be transmitted in each of the three consecutive PRBpairs respectively, or they may follow any other predetermined order.Both eNB and UE may follow an implicit rule to determine the DMRSpattern used for a particular PRB pair. It should be noted that suchimplicit association may apply when the PRB pair is assigned to a newUE, which supports such new DMRS patterns. For those PRB pairs assignedto legacy UE, which does not support such new DMRS patterns, the DMRSpatterns specified in TM9 shall still be used. While DMRS patterns Alt5-1, 5-2, and 5-3 are used in the example above, it should be understoodthat any of the DMRS patterns described herein may be used.

FIG. 21 is a diagram of DMRS pattern association with PRB bundling 2100.PRB pair #0 2105 may use DMRS pattern Alt 5-1, PRB pair #1 2110 may useDMRS pattern Alt 5-2, and PRB pair #2 2115 may use DMRS pattern Alt 5-3.As described above, this association may be repeated for subsequent PRBpairs. If a new UE is scheduled using PRB pair #n+1 2120, PRB pair #n+22125, and PRB pair #n+3 2130, the PRB pairs may use DMRS pattern Alt5-2, DMRS pattern Alt 5-3, and DMRS pattern Alt 5-1, respectively. TheUE may assume the DMRS patterns to use implicitly based on the PRB indexor the DMRS patterns may be explicitly provided to the UE.

The above described DMRS patterns associated with a UE may be eitherfixed or configurable. For example, the DMRS pattern configuration maybe dynamic, e.g., signaled to the UE as part of the DL grant or the DMRSpattern configuration may be signaled to the UE semi-statically throughbroadcasting or higher layer signals like RRC. If the DMRS patternconfiguration is RRC signaled, it may be UE-specific or cell-specific.

The DMRS configuration may be selected based on various factors. Forexample, if a UE is stationary, or with very low mobility, it may beconfigured with a DMRS pattern with low density in the time domain, e.g.DMRS pattern Alt 1. As another example, if the UE has medium mobility,it may be configured with DMRS pattern with low density in frequencydomain but relatively higher density in time domain, e.g. DMRS patternAlt 4 and DMRS pattern Alt 5. As some DMRS patterns may only support alimited number of DMRS ports such as DMRS pattern Alt 1, DMRS patternAlt 2, and DMRS pattern Alt 3, the configuration of DMRS patterns mayalso depend on the number of layers that an eNB may schedule to the UE,or the number of layers that a UE is capable to support.

In some embodiments, if the UE is semi-statically configured, newtransmission modes which may be associated with new DMRS patternsdescribed herein may not be required. For example, if the UE is a legacyUE, it will continue to use the TM 9 DMRS pattern. If the UE is a new UEand configured with one or more of the DMRS patterns described herein,it will automatically assume those DMRS patterns during scheduled PDSCHtransmissions.

As described above, small cells may be deployed for hot spots and indoorenvironments. As the coverage of small cell is quite small physically, acluster of small cells may be deployed together to cover a larger area.In addition, it may be preferred that minimum planning effort berequired for small cell deployment. Clustered deployment of small cellsmay lead to coverage overlaps among small cells and thus potentiallycause strong interference among small cells. DMRS defined in TM9 may betransmitted in the same REs in a PRB pair from all the cells, therebycolliding with each other. The collisions may degrade the channelestimation performance and thereafter the system throughput. To avoidsuch degradation, it may be beneficial to use orthogonal REs for DMRStransmission among small cells. For example, DMRS patterns Alt 2 and Alt3 may be used in overlapping small cells, in this case the differentDMRS patterns may avoid DMRS collision with each other and thereforelead to improved channel estimation performance. This embodiment may bedescribed as DMRS flipping, which may be considered as a special case ofshifting. Alternatively, DMRS transmission from three small cells mayuse DMRS patterns Alt 5-1, Alt 5-2, and Alt 5-3 respectively; in thiscase, the DMRS from these cells won't collide with each other and thusmaintain channel estimation performance from degradation. Furthermore,DMRS collision with PDSCH may be avoided by muting REs in a PRB pairfrom the serving cells that used to transmit DMRS patterns inneighboring cells. Such RE configuration may be signaled to the UE forit to use in rate matching.

In some embodiments, DMRS pattern shifting may be supported by smallcells coordinating with each other, the DMRS pattern may be explicitlysignaled among the small cells. For each small cell, its assigned DMRSpattern may be signaled to the UE, e.g., through RRC or broadcasting ofsystem information. Alternatively, each DMRS pattern may be assigned toa cell implicitly. For example, each DMRS pattern may be given an indexfrom the set of {0, 1, 2, . . . }, then the cell ID of a small cell maybe used to select the DMRS pattern used for the small cell, for exampleaccording to the following:

-   -   DMRS pattern index=mod(cell ID, N); Where N is the number of        DMRS patterns available.        At the UE side, the UE will determine the DMRS pattern        implicitly based on the cell ID as described herein.

While one simple formula is provided for implicitly determining a DMRSpattern, any number of methods and formulas may be used to implicitlydetermine a DMRS pattern.

In another embodiment, available DMRS patterns may be generated byshifting the existing DMRS pattern on the frequency domain. For example,the DMRS pattern Alt 2 may be used to generate at least 3 different DRMSpatterns by shifting along the frequency domain. When PRB bundling isapplied, the available DMRS patterns may be determined by the minimumbundling size as well. It is possible some patterns may have partialoverlap, but the neighboring small cells may be configured to preventsignificant DMRS overlapping among neighboring small cells.

In some embodiments, in small cells, DMRS may be used for PDSCHdemodulation, instead of using CRS as described in LTE rel-8. In thesecases CRS overhead may be reduced. However, minimal CRS may bemaintained in order to support UEs for decoding of legacy PDCCH and forchannel measurement like RRM. To reduce overhead and further improvesystem throughput, the number of CRS ports may be limited, for example,to only support 1 or 2 CRS ports in a small cell. In this case, CRSoverhead would be limited to less than 9%. Considering that the coverageof small cell is not large, 1 or 2 CRS ports may be enough to supportPDSCH for legacy UEs and legacy PDCCH. Further, the mobilitymeasurements (RRM) are mostly handled by the macro-cell, thus theaccuracy for the RRM within small cells, where UE mobility is minimal,may not be necessary to be as accurate as in macro-cells. Thus, CRSports may be reduced to save overhead. However, for legacy carriers thatneed to support legacy UEs on the small cells, minimal CRS may besupported. In certain embodiments, UEs may monitor the CRS of amacro-cell for the initial RRM, before attaching to a small cell. A flagmay be used within the small cell to indicate either “reduced” or“original” CRS usage. The flag may be part of the small cellconfiguration sent from a macro-cell.

In another embodiment, the CRS may be transmitted only on a subset ofsymbols/subframes and/or a subset of PRBs. For example, certainsubframes may be configured for CRS transmission. Such configuration maybe signaled through higher layer signals like RRC. Alternatively, alongthe frequency domain, some PRB pairs may be configured for CRStransmission, for example, some consecutive PRB pairs and distributedPRB pairs may be configured for CRS transmission. Such configuration mayalso be signaled to the UE semi-statically using higher layer signalslike RRC. Such configuration may be signaled in SIB.

In some embodiments, DMRS patterns with a reduced number of REs per PRBpair may be transmitted for PDSCH demodulation in a small cell. Tosupport this, a new transmit mode, e.g., TM 11, may be introduced tosupport PDSCH transmission in small cells. A new transmission mode mayalso come with a new DCI format (i.e., DCI format 2E), which may allowmore changes to satisfy the needs and characteristics of transmissionfor small cells. The new TM may also be configured to limit the changeof DMRS patterns, and therefore make the introduction of such newfeatures backward compatible to legacy UEs. Because TM 9 or TM 10 maystill be used for legacy UEs, an existing DMRS pattern for TM9 wouldstill be transmitted in PRBs scheduled for the legacy UE.

In some embodiments, small cells may be deployed in a cluster with aself-organizing capability, which means after the small cell is poweredup, the small cell may be able to recognize the presence of an overlaidmacro-cell and other neighboring small cells and synchronize with themin time and/or frequency. The small cell may then start to serve the UEsunder its coverage. Synchronization among the small cells and themacro-cell may improve interference coordination, network coordination,dual connections, etc. Synchronization may occur between the macro-celland small cell, and between small cells. The macro-cell and small cellsmay be synchronized at both radio frame and the OFDM symbol levels.

Several methods may be used for synchronization, for example, the cellsmay use an air-interface to enable the small cell to synchronize with amacro-cell or neighbouring small cells. After a small cell is deployed(supplied with power and connected with backhaul), in one alternative,it may start to listen to the macro-cell by detecting its primary andsecondary synchronization signals (PSS and SSS) similar to the way a UElistens to synchronization signals. When the macro-cell and small celloperate at different frequency bands, like one in 700 MHz, and anotherin 3.5 GHz, the small cell may need a separate RF chain to listen to themacro-cell downlink. After the small cell detects the PSS/SSS frommacro-cell, it may build up subframe synchronization with themacro-cell. The small cell may acquire and decode the physical broadcastchannel (PBCH) of the macro-cell for radio frame level synchronization.Further, time and frequency synchronization may be achieved throughdetecting the cell specific reference signals (CRS) or CSI-RS. The smallcell may then transmit its own PSS/SSS if needed.

After sync with the macro-cell, the small cell may still need to performsync tracking to maintain sync with the macro-cell. In order to maintainsync, the small cell may listen to the downlink from macro-cell or aneighbouring small cell for CRS or CSI-RS. There are several cases thatapply to the small cell listening to the downlink from the macro-cell ora neighbouring small cell, six cases are discussed below.

First, the macro-cell and small cell may operate on different frequencybands. In this case, the small cell may need a separate RF chain toreceive downlink information from the macro-cell. The small cell maythen use the CRS or CSI-RS transmitted in the macro-cell downlink to dosync tracking, i.e. to maintain synchronization over time. In this case,some coordination between the macro-cell and small cell is needed. Forexample, if the macro-cell operates on a legacy carrier, the small cellcould obtain the MBSFN configuration from macro-cell, and try to avoiddoing sync tracking in such subframes due to the lack of CRS. if themacro-cell operates on a NCT, the macro-cell should inform the smallcell of the subframe locations carrying the RS such as CRS, TRS orCSI-RS

Second, the macro-cell and small cell may operate on the same frequency(co-channel case). In this case, even if the small cell has a separateRF chain for listening to downlink from macro-cell, there can be aninterference issue in some scenarios if the small cell also transmits inits own downlink frequency band at the same time. This is because if notfiltered properly, the strong transmitted signal may leakage to thereceiver listening to the macro-cell downlink and cause interference asresult of third order inter-modulation product (IMP) due to anynon-linearity in the receiver. One solution is that the macro-cell couldperiodically transmit the sync information via a wire-line backhaul. Theperiodicity depends on the accuracy required for the sync tracking. Theother one solution is that the small cell does not transmit when it islistening to the macro-cell. FIG. 22 is a diagram of a macro-cell CRSpattern 2200. The macro-cell may transmit across all OFDM symbols in thePRB pair. FIG. 23 is a diagram of a small cell CRS pattern 2300 wherethe CRS is only transmitted in the first OFDM symbol in a subframe. Thesmall cell may be configured to only transmit in the first several OFDMsymbols of a subframe and listen to the macro-cell during the remainingOFDM symbols in the subframe. This may be achieved by configuring anMBSFN subframe in the small cell. In this case, after transmitting thefirst one or two OFDM symbols (i.e. a non-MBSFN region used to carryphysical control channels such as PCFICH, PHICH, PDCCH), the small cellmay then switch to listen to the downlink from macro-cell, as depictedin FIG. 23, and switch back before the start of the next subframe. Thisis possible because in a MBSFN subframe, the small cell does nottransmit CRS signals in the MBSFN region (i.e. the OFDM symbols otherthan the first one or two OFDM symbols in a subframe) and may notschedule any PDSCH in the subframe. This method of listening to themacro-eNB may be implemented under the existing 3GPP LTE spec and may betransparent to UE. To achieve this, some coordination is needed betweensmall cell and macro-cell and between small cells in a cluster. Forexample, the macro-cell should transmit normal subframes or ABS whichcontains CRS when a small cell is configured with MBSFN subframes forsynchronization tracking.

For a small cell cluster where a number of small cells are close to eachother, coordination is needed in the cluster so that all small cellsshould be configured with the same set of subframes as MBSFN. That wouldallow all small cells listen to macro-cell for synchronization tracking,and at the same time do not introduce interference to each other in suchsubframes. Alternatively, an anchor small cell in a cluster could beconfigured with MBSFN subframes for synchronization tracking with themacro-cell while the rest of small cells are also configured with MBSFNsubframes or subframes that they do not transmit anything (on a newcarrier), so that they do not cause interference to the anchor smallcell during its synchronization tracking. These small cells in a clustercould conduct synchronization and its tracking with the anchor smallcell. For example they (the rest of the small cells except the anchorsmall cell in a cluster) could be configured with another set ofsubframes which are different from those configured by the anchor smallcell for its sync tracking with the macro-cell, and conduct synctracking with the anchor small cell on these subframes at which theanchor small cell are supposed to transmit normal subframes or ABS whichcontains CRS. In the case that small cell uses new carrier type (NCT)which may not transmit CRS, the CSI-RS could be used for sync tracking.In this case, a small cell in a cluster, which is not an anchor smallcell, could be configured with MBSFN subframes or other new type ofsubframes that it does not need to transmit. Such subframes correspondto the subframes that the anchor small cell transmits CSI-RS. The smallcell could then use CSI-RS for sync tracking. Alternatively, the zeropower (ZP) CSI-RS could be configured in small cells in a cluster otherthan the anchor small cell, and REs of such ZP CSI-RS corresponds to REsthat non zero power (NZP) CSI-RS are transmitted in the anchor smallcell. That would allow the other small cells listen to anchor small cellon CSI-RS and use them for sync tracking. For this case, the small cellwould need a transceiver would could transmit and receive at the sametime.

Third, the macro-cell and small cell may operate on the same frequency(co-channel case) in TDD. In this case, the small cell may not need aseparate RF chain for listening to downlink from the macro-cell as bothdownlink and uplink are transmitted on the same frequency. In this casethe small cell and macro-cell may have the same DL/UL configuration sothat when the subframe is configured for downlink transmission in thesmall cell, the small cell may transmit PDCCH in the first severalsymbols and then listen to the downlink from the macro-cell in the restof the symbols. The listening operation by the small cell may betransparent to the UE served in small cell, by configuring an MBSFNsubframe in the small cell for this purpose. The small cell may transmitPDCCH in the first one or two symbols and then listen to downlink frommacro-cell. To reduce interference from other neighbouring small cellsthat may transmit while the small cell is listening to the macro-cell,some coordination may be need among small cells such that they maylisten to the macro-cell at the same subframes. To facilitate the synctracking for small cell, the macro-cell may transmit a normal subframe(or ABS) with CRS and to achieve this, some coordination may be neededamong small cell and macro-cell. If the Macro-cell and the small celluse different TDD DL/UL configuration, the small cell may listen to theMacro-cell for sync tracking in the subframes which are DL subframes forboth Macro and small cells.

Fourth, a small cell cluster may not see a macro-cell, for example, instandalone small cell deployment. In this case, one of the small cellsmay be configured as the timing reference and its sync rank status maybe broadcast to other neighbouring small cells. For example, the smallcell that equipped with a GPS could claim itself as a timing reference.The sync rank may be used to indicate a cell's timing accuracy, forexample, rank=1 may indicate the most accurate timing in a cell clusterand rank=2 may indicate a timing accuracy less than that of rank=1. Theneighbouring small cells may listen and try to synchronize with thesmall cell with the highest timing accuracy (i.e. lowest rank value) andin turn, may also broadcast their sync rank status with an increasedrank value. This sync rank information may be indicated in the PBCH anda small cell may need to decode PBCH of the cell which it is listeningto. Alternatively, RRC or SIB could be used to convey such information.For Sync tracking purpose, some coordination may be needed to make surethat the all small cells are not listening at the same time becauseotherwise there is no small cell transmitting at the time and nothing tolisten to. For example, the small cell with highest timing accuracy canbe configured to transmit while other small cells could be configured tolisten.

Fifth, some small cells in a cluster may observe the macro-cell, whileothers may not. In this case, the small cells which can observe theMacro-cell may first synchronize to the Macro-cell, and then, thesynchronized small cells may broadcast their synchronization rankstatus. Then the other small cells may synchronize to the synchronizedsmall cells and track the sync.

Sixth, if X2 interfaces exist, the small cell may use the proper framingalignment on the backhaul and exchange the timing offset for the radioframes to achieve synchronization.

Certain embodiments above may be implemented by a network element. Asimplified network element is shown with regard to FIG. 24. In FIG. 24,network element 3110 includes a processor 3120 and a communicationssubsystem 3130, where the processor 3120 and communications subsystem3130 cooperate to perform the methods described above.

Further, certain embodiments may be implemented by a UE. One exemplarydevice is described below with regard to FIG. 25. UE 3200 is typically atwo-way wireless communication device having voice and datacommunication capabilities. UE 3200 generally has the capability tocommunicate with other computer systems on the Internet. Depending onthe exact functionality provided, the UE may be referred to as a datamessaging device, a two-way pager, a wireless e-mail device, a cellulartelephone with data messaging capabilities, a wireless Internetappliance, a wireless device, a mobile device, or a data communicationdevice, as examples.

Where UE 3200 is enabled for two-way communication, it may incorporate acommunication subsystem 3211, including a receiver 3212 and atransmitter 3214, as well as associated components such as one or moreantenna elements 3216 and 3218, local oscillators (LOs) 3213, and aprocessing module such as a digital signal processor (DSP) 3220. As willbe apparent to those skilled in the field of communications, theparticular design of the communication subsystem 3211 will be dependentupon the communication network in which the device is intended tooperate.

Network access requirements will also vary depending upon the type ofnetwork 3219. In some networks network access is associated with asubscriber or user of UE 3200. A UE may require a removable useridentity module (RUIM) or a subscriber identity module (SIM) card inorder to operate on a network. The SIM/RUIM interface 3244 is normallysimilar to a card-slot into which a SIM/RUIM card can be inserted andejected. The SIM/RUIM card can have memory and hold many keyconfigurations 3251, and other information 3253 such as identification,and subscriber related information.

When required network registration or activation procedures have beencompleted, UE 3200 may send and receive communication signals over thenetwork 3219.

Signals received by antenna 3216 through communication network 3219 areinput to receiver 3212, which may perform such common receiver functionsas signal amplification, frequency down conversion, filtering, channelselection and the like. Analog to digital (A/D) conversion of a receivedsignal allows more complex communication functions such as demodulationand decoding to be performed in the DSP 3220. In a similar manner,signals to be transmitted are processed, including modulation andencoding for example, by DSP 3220 and input to transmitter 3214 fordigital to analog (D/A) conversion, frequency up conversion, filtering,amplification and transmission over the communication network 3219 viaantenna 3218. DSP 3220 not only processes communication signals, butalso provides for receiver and transmitter control. For example, thegains applied to communication signals in receiver 3212 and transmitter3214 may be adaptively controlled through automatic gain controlalgorithms implemented in DSP 3220.

UE 3200 generally includes a processor 3238 which controls the overalloperation of the device. Communication functions, including data andvoice communications, are performed through communication subsystem3211. Processor 3238 also interacts with further device subsystems suchas the display 3222, flash memory 3224, random access memory (RAM) 3226,auxiliary input/output (I/O) subsystems 3228, serial port 3230, one ormore keyboards or keypads 3232, speaker 3234, microphone 3236, othercommunication subsystem 3240 such as a short-range communicationssubsystem and any other device subsystems generally designated as 3242.Serial port 3230 can include a USB port or other port known to those inthe art.

Some of the subsystems shown in FIG. 25 perform communication-relatedfunctions, whereas other subsystems may provide “resident” or on-devicefunctions. Notably, some subsystems, such as keyboard 3232 and display3222, for example, may be used for both communication-related functions,such as entering a text message for transmission over a communicationnetwork, and device-resident functions such as a calculator or tasklist.

Operating system software used by the processor 3238 may be stored in apersistent store such as flash memory 3224, which may instead be aread-only memory (ROM) or similar storage element (not shown). Thoseskilled in the art will appreciate that the operating system, specificdevice applications, or parts thereof, may be temporarily loaded into avolatile memory such as RAM 3226. Received communication signals mayalso be stored in RAM 3226.

As shown, flash memory 3224 can be segregated into different areas forboth computer programs 3258 and program data storage 3250, 3252, 3254and 3256. These different storage types indicate that each program canallocate a portion of flash memory 3224 for their own data storagerequirements. Processor 3238, in addition to its operating systemfunctions, may enable execution of software applications on the UE. Apredetermined set of applications that control basic operations,including at least data and voice communication applications forexample, will normally be installed on UE 3200 during manufacturing.Other applications may be installed subsequently or dynamically.

Applications and software may be stored on any computer readable storagemedium. The computer readable storage medium may be a tangible or intransitory/non-transitory medium such as optical (e.g., CD, DVD, etc.),magnetic (e.g., tape) or other memory known in the art.

One software application may be a personal information manager (PIM)application having the ability to organize and manage data itemsrelating to the user of the UE such as, but not limited to, e-mail,calendar events, voice mails, appointments, and task items. Naturally,one or more memory stores may be available on the UE to facilitatestorage of PIM data items. Such PIM application may have the ability tosend and receive data items, via the wireless network 3219. Furtherapplications may also be loaded onto the UE 3200 through the network3219, an auxiliary I/O subsystem 3228, serial port 3230, short-rangecommunications subsystem 3240 or any other suitable subsystem 3242, andinstalled by a user in the RAM 3226 or a non-volatile store (not shown)for execution by the processor 3238. Such flexibility in applicationinstallation increases the functionality of the device and may provideenhanced on-device functions, communication-related functions, or both.For example, secure communication applications may enable electroniccommerce functions and other such financial transactions to be performedusing the UE 3200.

In a data communication mode, a received signal such as a text messageor web page download will be processed by the communication subsystem3211 and input to the processor 3238, which may further process thereceived signal for output to the display 3222, or Alternatively to anauxiliary I/O device 3228.

A user of UE 3200 may also compose data items such as email messages forexample, using the keyboard 3232, which may be a complete alphanumerickeyboard or telephone-type keypad, among others, in conjunction with thedisplay 3222 and possibly an auxiliary I/O device 3228. Such composeditems may then be transmitted over a communication network through thecommunication subsystem 3211.

For voice communications, overall operation of UE 3200 is similar,except that received signals may typically be output to a speaker 3234and signals for transmission may be generated by a microphone 3236.Alternative voice or audio I/O subsystems, such as a voice messagerecording subsystem, may also be implemented on UE 3200. Although voiceor audio signal output is preferably accomplished primarily through thespeaker 3234, display 3222 may also be used to provide an indication ofthe identity of a calling party, the duration of a voice call, or othervoice call related information for example.

Serial port 3230 in FIG. 25 may normally be implemented in a personaldigital assistant (PDA)-type UE for which synchronization with a user'sdesktop computer (not shown) may be desirable, but is an optional devicecomponent. Such a port 3230 may enable a user to set preferences throughan external device or software application and may extend thecapabilities of UE 3200 by providing for information or softwaredownloads to UE 3200 other than through a wireless communicationnetwork. The Alternate download path may for example be used to load anencryption key onto the device through a direct and thus reliable andtrusted connection to thereby enable secure device communication. Aswill be appreciated by those skilled in the art, serial port 3230 canfurther be used to connect the UE to a computer to act as a modem.

Other communications subsystems 3240, such as a short-rangecommunications subsystem, is a further optional component which mayprovide for communication between UE 3200 and different systems ordevices, which need not necessarily be similar devices. For example, thesubsystem 3240 may include an infrared device and associated circuitsand components or a Bluetooth™ communication module to provide forcommunication with similarly enabled systems and devices. Subsystem 3240may further include non-cellular communications such as WiFi or WiMAX.

The UE and other components described above might include a processingcomponent that is capable of executing instructions related to theactions described above. As used herein, the term instructions mayinclude reserved words which cause one or more processors to takecertain computational, memory-related or control actions or to sendcomputational, memory-related or control signals. As used herein, theterm program may include a collection of computer instructions. FIG. 26illustrates an example of a system 3300 that includes a processingcomponent 3310 suitable for implementing one or more embodimentsdisclosed herein. The processing component 3310 may be substantiallysimilar to the processor 3120 of FIG. 24 and/or the processor 3238 ofFIG. 25.

In addition to the processor 3310 (which may be referred to as a centralprocessor unit or CPU), the system 3300 might include networkconnectivity devices 3320, random access memory (RAM) 3330, read onlymemory (ROM) 3340, secondary storage 3350, and input/output (I/O)devices 3360. These components might communicate with one another via abus 3370. In some cases, some of these components may not be present ormay be combined in various combinations with one another or with othercomponents not shown. These components might be located in a singlephysical entity or in more than one physical entity. Any actionsdescribed herein as being taken by the processor 3310 might be taken bythe processor 3310 alone or by the processor 3310 in conjunction withone or more components shown or not shown in the drawing, such as adigital signal processor (DSP) 3380. Although the DSP 3380 is shown as aseparate component, the DSP 3380 might be incorporated into theprocessor 3310.

The processor 3310 executes instructions, codes, computer programs, orscripts that it might access from the network connectivity devices 3320,RAM 3330, ROM 3340, or secondary storage 3350 (which might includevarious disk-based systems such as hard disk, floppy disk, or opticaldisk). While only one CPU 3310 is shown, multiple processors may bepresent. Thus, while instructions may be discussed as being executed bya processor, the instructions may be executed simultaneously, serially,or otherwise by one or multiple processors. The processor 3310 may beimplemented as one or more CPU chips and may be a hardware devicecapable of executing computer instructions.

The network connectivity devices 3320 may take the form of modems, modembanks, Ethernet devices, universal serial bus (USB) interface devices,serial interfaces, token ring devices, fiber distributed data interface(FDDI) devices, wireless local area network (WLAN) devices, radiotransceiver devices such as code division multiple access (CDMA)devices, global system for mobile communications (GSM) radio transceiverdevices, universal mobile telecommunications system (UMTS) radiotransceiver devices, long term evolution (LTE) radio transceiverdevices, worldwide interoperability for microwave access (WiMAX)devices, and/or other well-known devices for connecting to networks.These network connectivity devices 3320 may enable the processor 3310 tocommunicate with the Internet or one or more telecommunications networksor other networks from which the processor 3310 might receiveinformation or to which the processor 3310 might output information. Thenetwork connectivity devices 3320 might also include one or moretransceiver components 3325 capable of transmitting and/or receivingdata wirelessly.

The RAM 3330 might be used to store volatile data and perhaps to storeinstructions that are executed by the processor 3310. The ROM 3340 is anon-volatile memory device that typically has a smaller memory capacitythan the memory capacity of the secondary storage 3350. ROM 3340 mightbe used to store instructions and perhaps data that are read duringexecution of the instructions. Access to both RAM 3330 and ROM 3340 istypically faster than to secondary storage 3350. The secondary storage3350 is typically comprised of one or more disk drives or tape drivesand might be used for non-volatile storage of data or as an over-flowdata storage device if RAM 3330 is not large enough to hold all workingdata. Secondary storage 3350 may be used to store programs that areloaded into RAM 3330 when such programs are selected for execution.

The I/O devices 3360 may include liquid crystal displays (LCDs), touchscreen displays, keyboards, keypads, switches, dials, mice, track balls,voice recognizers, card readers, paper tape readers, printers, videomonitors, or other well-known input/output devices. Also, thetransceiver 3325 might be considered to be a component of the I/Odevices 3360 instead of or in addition to being a component of thenetwork connectivity devices 3320.

The embodiments described herein are examples of structures, systems ormethods having elements corresponding to elements of the techniques ofthis application. This written description may enable those skilled inthe art to make and use embodiments having alternative elements thatlikewise correspond to the elements of the techniques of thisapplication. The intended scope of the techniques of this applicationthus includes other structures, systems or methods that do not differfrom the techniques of this application as described herein, and furtherincludes other structures, systems or methods with insubstantialdifferences from the techniques of this application as described herein.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods may beembodied in many other specific forms without departing from the scopeof the present disclosure. The present examples are to be considered asillustrative and not restrictive, and the intention is not to be limitedto the details given herein. For example, the various elements orcomponents may be combined or integrated in another system or certainfeatures may be omitted, or not implemented.

Also, techniques, systems, subsystems and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component, whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and Alterations are ascertainable by one skilled in theart and may be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A system comprising: a first network element; anda second network element, the first network element comprising aprocessor configured to: synchronize with the second network element;and maintain synchronization with the second network element, whereinthe first network element is a small cell eNB and the second networkelement is one of the following: a macro cell enhanced node-B (eNB); ora small cell eNB.
 2. The system of claim 1, wherein the first networkelement and the second network element both communicate on a frequencyband, and wherein the first network element transmits on a first portionof a subframe and listens to a downlink subframe from the second networkelement on a second portion of the subframe to maintain synchronization.3. The system of claim 2, wherein the downlink subframe from the secondnetwork element comprises reference signals selected from one or more ofthe following: Cell-specific reference signal (CRS); Channel stateinformation-reference signal (CSI-RS); PSS/SSS; and Tracking referencesignals (TRS).
 4. The system of claim 2, wherein the first networkelement and the second network element both communicate on a frequencyband using time division duplexing (TDD).
 5. The system of claim 4,wherein a downlink/uplink configuration of the first network element isthe same as that of the second network element.
 6. The system of claim1, wherein when the second network element is a macro cell eNB, and thefirst network element is unable to synchronize with the second network,the first network element synchronizes with a neighboring small celleNB.
 7. The system of claim 6, wherein the neighboring small cell is ananchor cell.
 8. The system of claim 6, wherein the neighboring smallcell is able to synchronize with the second network element.
 9. Thesystem of claim 1, wherein the first network element and the secondnetwork element both communicate on an X2 interface, the first networkelement and the second network element align frames and exchange atiming offset over the X2 interface.
 10. A method for synchronizationcomprising: synchronizing a first network element with a second networkelement; and maintaining synchronization with the second networkelement, wherein the first network element is a small cell eNB and thesecond network element is one of the following: a macro cell enhancednode-B (eNB); or a small cell eNB.
 11. The method of claim 10, whereinthe first network element and the second network element bothcommunicate on a frequency band, and wherein the first network elementtransmits on a first portion of a subframe and listens to a downlinksubframe from the second network element on a second portion of thesubframe.
 12. The method of claim 11, wherein the downlink subframe fromthe second network element comprises reference signals selected from oneor more of the following: Cell-specific reference signal (CRS); Channelstate information-reference signal (CSI-RS); PSS/SSS; or Trackingreference signals (TRS).
 13. The method of claim 10, wherein the firstnetwork element and the second network element both communicate on afrequency band using time division duplexing (TDD).
 14. The method ofclaim 13, wherein a downlink/uplink configuration of the first networkelement is the same that of the second network element.
 15. The methodof claim 10, wherein when the second network element is a macro celleNB, and the first network element is unable to synchronize with thesecond network, the first network element synchronizes with aneighboring small cell eNB.
 16. The method of claim 15, wherein theneighboring small cell is an anchor cell.
 17. The method of claim 15,wherein the neighboring cell is able to synchronize with the secondnetwork element.
 18. The method of claim 10, wherein the first networkelement and the second network element both communicate on an X2interface, the first network element and the second network elementalign frames and exchange a timing offset over the X2 interface.
 19. Afirst network element comprising a processor configured to: synchronizewith a second network element; and maintain synchronization with thesecond network element, wherein the first network element is a smallcell eNB and the second network element is one of the following: a macrocell enhanced node-B (eNB); or a small cell eNB.
 20. The first networkelement of claim 19, wherein the first network element is furtherconfigured to: communicate on a frequency band, wherein the secondnetwork element also communicates on the frequency band, and transmit ona first portion of a subframe and listen to a downlink subframe from thesecond network element on a second portion of the subframe to maintainsynchronization.
 21. The first network element of claim 20, wherein thedownlink subframe from the second network element comprises referencesignals selected from one or more of the following: Cell-specificreference signal (CRS); Channel state information-reference signal(CSI-RS); PSS/SSS; or Tracking reference signals (TRS).
 22. The firstnetwork element of claim 20, wherein the first network element isconfigured to communicate on a frequency band using time divisionduplexing (TDD).
 23. The first network element of claim 22, wherein adownlink/uplink configuration of the first network element is the sameas that of the second network element.
 24. The first network element ofclaim 19, wherein when the second network element is a macro cell eNB,and the first network element is unable to synchronize with the secondnetwork, the first network element is configured to synchronize with aneighboring small cell eNB.
 25. The first network element of claim 24,wherein the neighboring small cell is an anchor cell.
 26. The firstnetwork element of claim 24, wherein the neighboring small cell is ableto synchronize with the second network element.
 27. The first networkelement of claim 19, wherein the first network element and the secondnetwork element are configured to communicate on an X2 interface, thefirst network element and the second network element are configured toalign frames and exchange a timing offset over the X2 interface.